1. Field of the Invention
The present invention relates to a high molecular nanocomposite membrane for a Direct Methanol Fuel Cell (DMFC), which includes a Nafion® high molecular membrane in which hydrophobic silica nanoparticles are dispersed, and a membrane-electrode assembly and a methanol fuel cell including the same membrane.
2. Description of Related Art
A fuel cell is an electrochemical energy converting device that directly converts the chemical energy of fuel into electrical energy through an electrochemical reaction. Compared to combustion engines, fuel cells exhibit high energy conversion efficiency and make very little noise during operation. Since fuel cells are an environmentally friendly power supply device since they do not produce pollutants, such as nitrogen oxides (NOx) and sulfur oxides (SOx), they are attracting a lot of interest and research.
Nowadays, in response to the increasing use of portable devices, such as notebook computers, and the trend toward high performance and multi-functionality, existing power supply devices are encountering limitations, and the development of a novel portable power supply device that exhibits high capacity and a small size is required. A Direct Methanol Fuel Cell (DMFC), which uses methanol as fuel, has advantages in that methanol has high volumetric energy density and in that it can stably store fuel and be easily transported. Under the current circumstances, in which the infrastructure for the supply of hydrogen has not been established, the simple fuel supply system of the DMFC, in which liquid methanol can be directly supplied to an anode without having to reform hydrogen from other types of fuel, as well as the simple structure of the entire system, enables the DMFC to be used as a small-size portable power supply device. In addition, since the DMFC not only operates at room temperature, but also can be miniaturized and sealed, it can be used for various applications, such as nonpolluting vehicles, home power generating systems, mobile communication devices, medical instruments, military equipment, equipment for the space industry, and mobile electronics.
The DMFC uses an aqueous methanol solution as fuel, and the swelling of a solid high molecular membrane occurs depending on the concentration of methanol in the aqueous methanol solution. The swelling is a phenomenon that occurs when the aqueous methanol solution is used as fuel, in which a portion of the fuel that is not oxidized by an electrochemical reaction permeates into the high molecular membrane to migrate from an anode to a cathode, thereby being wasted and causing a deterioration in performance at the cathode due to mixed potential. In order to solve this problem, the development of a DMFC-dedicated solid high molecular membrane is required.
Methods of reducing the permeability of the aqueous methanol solution using solid high molecules or using heat-resistant high molecules in the manufacture of the DMFC-dedicated solid high molecular membrane have been proposed (U.S. Pat. Nos. 5,795,496, 6,194,474, and 6,510,047). Although these methods can significantly decrease the permeation of methanol, they have a problem in that the ion conductivity of the high molecular membrane greatly decreases, thereby causing a deterioration in performance, such as reduced output density.
As another method of manufacturing a DMFC-dedicated solid high molecular membrane, a method of dispersing inorganic particles in the high molecular membrane was proposed (U.S. Pat. No. 5,919,583). However, this method has a problem in that the inorganic material that is added decreases the conductivity of hydrogen ions.
Although methanol has high volumetric energy density, the electrochemical reactivity of electrodes is low. Therefore, DMFC requires the use of a greater amount of expensive platinum (Pt) catalyst, which is required for the acidification of methanol, compared to a Polymer Electrolyte Membrane Fuel Cell (PEMFC). In addition, the catalyst has to be used in the form of an alloy combined with other metal elements, such as ruthenium (Ru), in order to raise the activity of the catalyst, since an unstable product such as CO or HCO, which is produced during the oxidation of methanol, causes catalyst poisoning. In addition, methanol crossover occurring when methanol migrates from the anode to the cathode through the high molecular membrane used as electrolyte decreases the difference in potential between the anode and the cathode, thereby causing a deterioration in the performance of the cell and a partial loss of fuel. For these reasons, at present, a diluted aqueous methanol solution having a concentration of 3 wt % to 5 wt % is generally used, and this is the main factor that reduces the performance of the fuel cell.
In addition, the high molecular electrolyte membrane that constitutes the fuel cell is an electronic insulator but conducts hydrogen ions. In general, the thinner the membrane is, the smaller the overvoltage due to Ohmic voltage drop (or IR drop) becomes. Also, the lower the equivalent weight is, the better the ion exchange performance becomes. So, the high molecular electrolyte membrane that is thick and has a low equivalent weight can be determined to be an electrolyte membrane having preferable properties. However, if the membrane is too thin, mechanical strength becomes weak, and a crossover phenomenon, in which gases supplied to both electrodes migrate to the counterpart electrodes through the membrane, may occur, thereby reducing the performance of the membrane. If the equivalent weight is too small, flooding may occur, thereby reducing the performance of the membrane. As is reported by Narayanon et al., although the electrolyte membrane itself is advantageous if the equivalent weight is small, the activity of the membrane may be reduced by flooding if a material that has a small equivalent weight is used for the electrolyte membrane used in the electrode. Therefore, the high molecular electrolyte membrane is advantageous if the equivalent weight is great.
At present, a Nafion® high molecular membrane is commercially distributed as a hydrogen ion-conductive high molecular membrane. However, the crossover of reaction fuel in the Nafion® high molecular membrane is a major obstacle that makes the DMFC difficult to be commercially distributed. The crossover of reaction fuel is a phenomenon, in which fuel migrates from a fuel electrode (negative electrode) to an air electrode (positive electrode) instead of being completely oxidized when the fuel is supplied to the fuel electrode. Such methanol crossover is the toughest challenge to overcome in the methanol fuel cell. In the present circumstances, the crossover causes fuel loss of 20% or more and voltage loss of 0.1V or more.
The information disclosed in this Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.